It is very important to know chemical forms and concentration of heavy metals in fly ash from municipal solid waste incinerator in order to detoxify and reuse it or clarify complex reaction mechanism such as de novo synthesis for dioxin on it. Chemical forms of heavy metals were estimated using X-ray Photoelectron Spectroscopy (XPS), X-ray diffraction, sequential extraction procedure and thermo-dynamics study. Zinc, copper, lead and cadmium were measured for its toxicity and concentration in fly ash.The application of X-ray diffraction analysis for heavy metals in fly ash was not effective to estimate chemical forms. Chemical forms of heavy metals in many kinds of fly ash were estimated by XPS, because heavy metals concentrated onto its surface and higher intensity was obtained for XPS.Sequential extraction procedure was used for the purpose of grasping the relationship between heavy metals and the composition in fly ash keeping heavy metals.Putting the results by each analysis together, the dominant zinc compounds in fly ash were 2ZnCO_3 3Zn(OH)_2, ZnSO_4 7H_2O and ZnO.Many compounds such as PbCl_2, PbCO_3, PbO, etc., were estimated and it is impossible to determine a dominant form in it. The dominant copper compound was CuO.Chemical form of cadmium in fly ash of dry process flue-gas treatment (Cd(OH)_2, CdO) was different from that of wet process flue-gas treatment (CdSO_4, CdCl_2).Estimation of chemical form of heavy metals in fly ash is possible by XPS and sequential extraction procedure respectively, but it is considered to be able to increase the exactitude of estimation by the combination of both methods.